1. Field of the Invention
This invention relates generally to a frequency harmonic comb generator and, more particularly, to a frequency harmonic comb generator that is tunable to vary the pulse width of the pulses in the output signal to provide maximum power for different harmonics.
2. Discussion of the Related Art
Comb frequency generators are well known devices that generate harmonics of a sinusoidal input signal. A comb frequency generator has many applications, including being used in frequency multipliers, local oscillators, and frequency synthesizers. The output signal of the comb generator is typically a series of narrow pulses, or impulses, that are periodic with the frequency of the input signal, where the output signal looks similar to the teeth of a comb and has a higher frequency than the input signal. The shape of the pulses defines the relative amplitudes and phases of the various harmonics in the output signal. Bandpass filters are used to filter out the undesirable harmonic pulses to select the desirable harmonics for a particular use. The selected harmonics can then be used, for example, as a modulation or demodulation carrier frequency. Multiple harmonics can be selectively filtered to provide a multi-channel device.
Comb generators typically convert the sinusoidal input signal to a signal having fast edges, i.e., pulses having very short rise or fall times. It is these fast edges or transitions that create the harmonics of the input signal. Various comb generators known in the art use comparators and limiting amplifiers to generate the fast edges. For those applications that require higher harmonics, state of the art comb generators typically employ step recovery diodes. The pulse width of the output signal is defined by the diode parameters of the step recovery diodes in the circuit configuration. The step recovery diodes hold an applied charge up to a certain voltage level, and then quickly release the charge to generate the fast edges. A discussion of using step recovery diodes for this purpose can be found in the document Harmonic Generation using step recovery diode and SRD module, Hewlett Packard.
Known comb generators have also employed non-linear transmission line (NLTL) wavefront compression devices to generate a signal having fast edges. The NLTL is typically a transmission line periodically loaded with varactor-type diodes that define an LC circuit. A discussion of NLTL wavefront compression used for this purpose can be found in the article, Case, Michael et al. “Picosecond duration, large amplitude impulse generation using electrical soliton effects,” Appl. Phys. Lett. 60 (24), 15 Jun. 1992, pgs. 3019–3021. A Schottky-contact microstrip line (SCML) is disclosed in the article Jäger, Dieter, “Slow-Wave Propagation Along Variable Schottky-Contact Microstrip Line,” IEEE Transactions On Microwave Theory And Techniques, Vol. MTT-24, No. 9, September 1976, pgs. 566–573. The SCML also generates a signal having fast edges that can be used in a comb generator. The SCML is a microstrip line formed on a semiconducting substrate including periodic Schottky-barrier diodes.
A disadvantage exists with the known comb generators because the width of the pulses in the output signal is fixed and is not variable. The output power of the generator for any selected harmonic can be maximized by carefully defining the pulse width of the harmonics. Only a subset of the harmonic frequencies is maximized for any given pulse width. In other words, only one of the harmonics in the output signal will have maximum power for a particular pulse width out of all of the pulses in the output signal. This disadvantage is a significant problem for those applications where different harmonics may be selected at different times during operation of the generator or for multi-channel devices.
Square pulses of amplitude A and width β will have a spectral envelope defined by Aβ|sin(πfβ)|/(πfβ), which is zero at frequencies that are even integer multiples of 1/(2β) and has an upper bound of A/(πf) at frequencies that are odd multiples of 1/(2β). The spectral envelope is a Sinx/x function that is defined by the fourier transform of a pulse. In known comb generators, the pulse width is typically made narrow, i.e., 1/(βFi) is greater than N, where N is the desired output harmonic and Fi is the frequency of the input signal, so that there are many harmonics between Fi and 1/β. Wider pulse widths can also be chosen that still maximize the power in the desired Nth harmonic such that the pulse width is an odd multiple of 1/(2NFi). This is useful when extremely narrow pulse widths are impractical, or when it is desirable to locate the nulls in the spectral envelope at the N−1 and N+1 harmonics to simplify the filtering requirements. An example would be setting β=1/(2Fi) which places the nulls of the spectral envelope at all the even harmonics of the input frequency.
FIG. 1 is a graph with frequency on the horizontal axis and amplitude on the vertical axis. A series of impulses 10 are shown that identify the harmonic frequencies in the output signal. A spectral envelope 12 identifies the Sin x/x function of the Fourier transform of the pulses. By varying the pulse width of the pulses, the lobes of the envelope 12 change. For example, when the pulse widths get narrower, the lobes of the spectral envelope 12 get wider, and vice versa. This changes the null locations between the lobes, relative to frequency. A curve, referred to herein as a meta-envelope 14, is a decaying exponential-type function that contacts each lobe at one location, as shown. The meta-envelope 14 represents the maximum output power that can be achieved for a particular amplitude input signal. The meta-envelope 14 is the same for a particular input frequency. As the pulse width changes, the location where the meta-envelope 14 contacts the lobe changes relative to the null locations. It is desirable to have the meta-envelope 14 contact the lobes half-way between the null locations for maximum power.
What is needed is a comb frequency generator that is tunable to vary the pulse width of the output signal to maximize the power for a selected harmonic. It is therefore an object of the present invention to provide such a tunable comb generator.